This invention relates to a method of etching semiconductor materials and, more particularly, to an improved method of etching grating patterns into epitaxial material.
In order to reduce signal distortion in optical communications systems due to the different propagation rates of different frequencies, sources capable of emitting single-frequency signals are of primary interest. This condition is imperative in the case of high-speed direct source modulation and for use in coherent communications systems.
Semiconductor lasers in which optical feedback is obtained by mechanisms other than multiple reflections between the mirrors delimiting a laser cavity are common examples of sources meeting the above requirement. In these lasers, a selection of the oscillation modes is obtained without resorting to external components so that the laser can be fabricated by conventional integrated-optics circuit technology. Moreover, owing to the absence of the end mirrors, the devices are suitable for integration with other components of an optical communications system.
As a result, semiconductor lasers play a crucial role as light sources for optical communication systems. By selecting appropriate semiconductor materials, conventionally III-V alloy compounds, lasers emitting in the 0.8-1.7 um wavelength range may be fabricated. At present, long haul optical fiber communications is based on operating wavelengths near 1.55 um and 1.3 um; the wavelengths at which single mode optical fibers have minimum attenuation and dispersion, respectively. Lasers have advantages over light emitting diodes (LEDs) of providing a coherent, narrow bandwidth source, ideally suited for communications applications. Single-mode, narrow linewidth light sources for optical communication are thus dominated by laser diodes fabricated from direct band gap, III-V semiconductor alloy materials, particularly InP/InGaAsP, which emit in this particular wavelength region. Shorter wavelength sources, e.g. GaAs/AlGaAs which emit light near 0.9 um, have traditionally been used for short distance transmission.
A semiconductor laser is ordinarily made of Group III-V semiconductor materials because the conditions for stimulated emission are easily obtained in these materials by suitable pump means. One particularly useful form of such a laser has distributed feedback (DFB) i.e. optical feedback is built into the laser cavity along its cavity length. Such feedback is supplied by means of a corrugated DFB waveguide grating. The grating lines run perpendicular to the propagation axis of the laser cavity.
Corrugated waveguide gratings are, in general, key elements in many optical components e.g. filters, distributed feedback (DFB) lasers, and distributed Bragg reflector (DBR) lasers. Such components will continue to play a significant role in future lightwave communication systems. As such, the fabrication of corrugated waveguide gratings continues to be a subject of great interest.
Advanced techniques of epitaxially growing semiconductor materials, for example, metal organic chemical vapor deposition (MOCVD), has led to the development of new active device structures such as multilayer configurations incorporating multi-quantum-wells and asymmetrical confinement layers. These developments have, in turn, resulted in the realization of surface or buried corrugated waveguide gratings in Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) lasers.
Optoelectronic devices with regular corrugation or gratings often require precise control of the corrugation or grating. In many DFB or DBR lasers, for example, the corrugation or grating must be etched into the epitaxial structure to produce a periodic modulation of the index of refraction of the laser cavity. A special case of a DFB or DBR laser is a laser with a gain-coupled grating. A gain-coupled grating penetrates the quantum wells in a multiple quantum well, or single quantum well, structure. In all cases, the etching of the corrugation or grating must be very well controlled to accurately determine the electrical and optical properties of the device, including, but not limited to, the mode stability, the lasing efficiency, the optical loss, the optical confinement, the modulation speed, the local current density and the heat conductivity.
The most common procedure for forming gratings in III-IV materials begins with patterning photoresist. The two most common ways to expose the photoresist are patterning by exposure to an interference pattern (such as a hologram) and patterning with electron-beam lithography. The pattern can be transferred from the photoresist to the III-IV material by etching in a wet chemistry. For example, InP-based materials can be etched with a solution of hydrobromic (HBr) and nitric acid (HNO3), or with a solution of iodic acid, though many other chemistries are also known. Another way to transfer the pattern is reactive ion etching (RIE). A major drawback of RIE is that it can cause damage to the crystalline structure which, if in the vicinity of the active region of the device, can be catastrophic. For this reason, in many applications specific to DFB and DBR lasers, wet etching is the preferred means of transferring the corrugation of grating pattern from the photoresist to the III-IV material.
With conventional etching techniques and processes, the grating pattern must be etched through materials with differing compositions and hence, different etch rates. For example, for many wet etch chemistries, quaternary (Q) material with Ga and As etches more quickly than pure InP. Semiconductor laser structures usually have the Ga and As containing layers beneath the InP layers to achieve the needed waveguiding. This means that a gain-coupled grating must be etched through the slow etch rate material into high etch rate material, leading to very poor depth control. To see why this leads to poor depth control, consider a grating tooth that gets slightly ahead of its neighbors while etching the InP. It will reach the quaternary first and could etch through many quantum wells before its neighbor even begins to attack the quaternary. If the target grating depth is deep in the active region, the neighboring grating teeth have a chance to catch up. However, if the target depth is near the top of the active region, very poor depth uniformity will be the result. Changes to etch conditions (concentration, choice of acids etc.) has not proven to greatly improve grating uniformity to date.
In many current DFB and DBR laser designs, weak gain-coupling is appropriate, therefore requiring a grating that penetrates only part-way into the active region. In such cases, the known art of direct wet etching, as described, results in poor depth control. Furthermore, with conventional etching techniques using a photoresist mask, the final etched grating is exposed to the photoresist and corresponding stripping solvent. This is particularly of concern for gain-coupled gratings, where the solvent may introduce contaminants directly into the active region of the device.
The present invention discloses an improved method of etching gratings for distributed feedback lasers. The method, however, could be applied more generally to etching other patterns into epitaxial material. The present invention uses an InP grating mask as a mask during grating etching. An epitaxial structure and process is described that allows the InP grating mask to be created from a pattern in photoresist. Furthermore, the process removes any InP that was in contact with contaminants like photoresist prior to final etching. The pattern in the InP grating mask is then transferred into underlying epitaxial material. Advantageously, the method of the present invention does not require exposing the final etched structure to solvents or other contaminants such as photoresist stripping solutions.
According to a broad aspect of the invention, then, there is provided a method for etching a pattern in a semiconductor material. The process begins with the formation of an InP grating mask on the semiconductor material, the InP grating mask selectively exposing the semiconductor material and defining the pattern to be etched. The exposed semiconductor material is then etched such that the pattern is transferred to the semiconductor material.
According to another broad aspect of the invention there is provided a method for etching a pattern in a semiconductor material as described above whereby the formation of the InP grating mask on the semiconductor material to be etched involves the design of a structure with an etch-stop layer between two InP layers. A photoresist grating mask corresponding to the pattern to be etched is formed on the top InP layer. A non-selective etch is then used to penetrate the top InP layer, the etch-stop layer, and the lower InP layer. A suitable stripping solvent is used to remove the photoresist followed by a selective etch which is used to clear the remaining exposed InP material, remove contaminated material and selectively expose the underlying semiconductor material in accordance with the pattern to be etched. Additional masking beyond the InP mask in not required.
The method of the present invention is based on removing the slow etch-rate material before etching into the high etch-rate material. This results in greatly improved uniformity, and greatly improved depth control. In the InP masking process of the invention, the solvents are applied during preparation of the InP grating mask; they are not applied after exposing the active region of the device.
The present invention, therefore, improves the yield of gain-coupled gratings in the fabrication of gain-coupled distributed feedback lasers. Advantageously, the invention may be applied to the fabrication of either buried heterojunction lasers or ridge waveguide lasers, two general categories of laser which are both important components of current optical communication systems.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.